The electronic configuration of gadolinium (Atomic No. 64) is:

The correct option is C $\left[Xe\right]4f^{7}5d^{1}6s^2$
The electronic configuration of $La(Z=57)$ is $\left[Xe\right]5d^{1}6s^2$. Therefore, further addition of electrons occurs in a lower energy 4f-orbital till it is exactly half filled at $Eu(Z=63)$. Thus,the electronic configuration of Eu is $\left[Xe\right]4f^{7}6s^2$. Therefore, addition of next electron doesn't occur in a more stable exactly half-filled $4f^7$ shell but occur in a little higher energy 5d-orbital. Thus, the electronic configuration of $Gd(Z=64)$ is $\left[Xe\right]4f^{7}5d^{1}6s^2$.